Selective Catalytic Reduction (SCR) systems find wide use in the power generation industry, where they are used for de-nitrification of the flue gas produced in coal-fired power generation. In a common arrangement, flue gas is directed downward through a catalyst layer or bed that is positioned normal to the downward flow of flue gas.
A reagent, such as ammonia, generally is injected into the flow upstream of the catalyst layer, and proper operation of the SCR system depends on achieving uniform flow conditions across the inlet face of the catalyst layer (or layers in a stacked catalyst system). Variances in flow velocity across the face of the catalyst layer impair the desired reaction process and lead to accelerated and uneven depletion of the catalyst layer.
Commonly, flue gas must be redirected for proper entry into an SCR system, which may be formed as a vertical enclosure. For example, flue gas may be horizontally ducted through a plenum into a top inlet of the vertical enclosure, and downwardly redirected into the enclosure. Without flow management provisions, such redirection causes flow disturbances and results in unbalanced flow conditions on the inlet face of the catalyst layer(s). A number of known flow redirecting devices find use in flue gas applications.
For example, U.S. Pat. No. 6,905,658 B2 to Rogers et al., discloses a device for redirecting flue gas via channeling. More particularly, the '658 patent teaches the formation of separate flow channels within a flue, for flow redirection and conditioning. Other known flow redirecting devices include turning vanes disposed within ductwork corners or at other junctions. Turning vanes offer good performance potential, but come at the expense of additional design, installation, and maintenance cost/complexity, given their curved shapes and other aerodynamic considerations.
As an alternative to controlled redirecting devices, it is also known to place flow conditioning devices downstream from redirection points, e.g., downstream from ducting corners. So-called rectifier or straightening grids represent a known example of downstream flow conditioning devices. A common straightening grid configuration involves placing a grid of parallel or crisscrossing blades edgewise in the redirected flow, to smooth the flow passing through the grid.
Further, in applications requiring very high flow quality—i.e., tight cross-sectional uniformity of flow velocity—it is known to use redirecting devices in combination with downstream conditioning devices. For example, turning vanes or very widely spaced blade systems may be mounted in a duct corner for controlled flow redirection, and a downstream straightening grid may be suspended within the redirect flow for further flow conditioning. While such combined arrangements are known to achieve potentially high flow quality, they can be disadvantageous in terms of cost and installation complexity. Additionally, practical constraints, such as plenum length, size, etc., may prevent or make impractical the use of both turning devices in combination with straightening grids to achieve high flow quality.